This invention relates to antennas and more particularly to microwave antennas that are used in a global positioning system.
A global positioning system (GPS) requires the use of a dual frequency circularly polarized antenna. GPS antennas range from fixed beam antennas to phased arrays to adaptive arrays. A common feature for all GPS antenna systems is an antenna element with the following requirements: dual frequency operation of a low frequency F.sub.l having a low central frequency of 1.227 Ghz, a high frequency F.sub.h having a high central frequency of 1.575 Ghz.
The bandwidth for both F.sub.l and F.sub.h is 20 Mhz. Polarization is right hand circular, axial ratio is not specified, the coverage is hemispherical, and the gain at 0.degree. is 0.0 dBci and at 80.degree. a -6.0 dBci.
Antenna elements that can meet these requirements are (1) dual frequency microstrip patch elements, (2) spiral elements and (3) volute elements. In some applications, there is an additional requirement for an antenna element with a low monostatic scattering in the region of 45.degree. to 90.degree. from zenith. Typical frequencies for the low monostatic scatterings are 500 Mhz to 18 Ghz.
The volute antenna element with a high profile has unacceptable monostatic scattering and therefore, does not satisfy the above stated requirements.
FIGS. 7 and 8 are illustrations showing a dual frequency microstrip patch antenna element of the prior art system and in particular a ground plane 7 is provided over which a low profile antenna element that includes a first microstrip patch 1 and a second microstrip patch 3 is mounted. Terminals 9 and 5 connect the dual frequencies between the antenna element and a receiver (not shown). In the embodiment of FIG. 8, the microstrip patch 1 is separated from the ground plane 7 by a honeycomb spacer 11. Similarly, the microstrip patch 3 is separated from the microstrip patch 1 via honeycomb spacer 13. The overall assembly is covered by a nonconductive radome 15.
The antenna element in FIGS. 7 and 8 has a moderate profile above the ground plane 7 and accordingly this antenna does not have low monostatic scattering.
The prior art spiral element antenna has marginal gain and does not meet the scattering requirement. A commonly used approach to reduce monostatic scattering for an antenna element is to recess the element into the ground plane and to cover the element with sufficient microwave absorbing material. However, this approach excessively reduces the antenna gain.